Typically, when mounting electronic components on a printed circuit board, cream solder is first printed on a predetermined electrode pattern formed on the printed circuit board. Next, based on the viscosity of the cream solder, the electronic components are temporarily mounted on the printed circuit board. Then, the printed circuit board is transferred to a reflow furnace, where it is soldered through a reflow process. Nowadays, the printed state of the cream solder must be inspected prior to entering the reflow furnace, where a three-dimensional measuring instrument is used for the cream solder inspection.
In recent years, various non-contact type three-dimensional measuring instruments using light have been proposed. One of them is a technology related to a three-dimensional measuring instrument using a phase shift method (Japanese Laid-Open Publication No. 11-211443 and Japanese Patent No. 2711042).
The three-dimensional measuring instrument based on the above technology uses a CCD camera. Namely, by an illuminating means formed of a combination of a light source and a filter with a sine wave pattern, the measuring instrument illuminates a light pattern having a striped light intensity distribution on the object of measurement (in this case, the printed circuit board). Then, it is observed by using a CCD camera placed right above the printed circuit board. In this situation, the light intensity I of a point P on the screen is expressed by the following formula.I=e+f·cos φ
[where, e is DC current light noise (offset component), f is sine wave contrast (reflection rate), and φ is phase based on the irregularity of the object)
At this point, by shifting the light pattern, the phase is changed to four steps (φ+0, φ+π/2, φ+π, φ+3π/2), and the images carrying the intensity distribution I0, I1, I2, and I3 corresponding to the four phase steps are retrieved so that positional information θ is determined by the following formula.θ=arc tan{(I3−I1)/(I0−I2)}
By using this positional information θ, the three-dimensional coordinates (X, Y, Z) of the point P on the printed circuit board (cream solder) is determined so that the three-dimensional shape of the cream solder, in particular, the height thereof, can be measured.
In the three-dimensional measuring instrument based on the above technology, the phase must be changed to four steps to obtain four images carrying the intensity distribution corresponding to each step. In other words, an image retrieval process has to be performed every time when the phase is changed, hence, requiring four image retrieval processes per point. Due to this, a long time is necessary for the image retrieval, which in turn causing a long overall measuring time from start to finish.
The present invention has been made by taking the above situation into consideration, where one of its objects is to provide a three-dimensional measuring instrument which is capable of shortening the measuring time required when measuring the three-dimensional shape of an object by the phase shift method.